Zodiac

The Earth in its orbit around the Sun causes the Sun to appear on the celestial sphere moving over the ecliptic (red), which is tilted with respect to the equator (blue-white).

In both astrology and historical astronomy, the zodiac (Greek: ζῳδιακός, zōidiakos) is a circle of twelve 30° divisions of celestial longitude that are centered upon the ecliptic, the apparent path of the Sun across the celestial sphere over the course of the year. The paths of the Moon and visible planets also remain close to the ecliptic, within the belt of the zodiac, which extends 8-9° north or south of the ecliptic, as measured in celestial latitude. Because the divisions are regular, they do not correspond exactly to the twelve constellations after which they are named.

The term zodiac derives from Latinzōdiacus, which in its turn comes from the Greekζῳδιακὸς κύκλος (zōdiakos kyklos), meaning "circle of animals", derived from ζῴδιον (zōdion), the diminutive of ζῷον (zōon) "animal". The name is motivated by the fact that half of the signs of the classical Greek zodiac are represented as animals (besides two mythological hybrids).

Although the zodiac remains the basis of the ecliptic coordinate system in use in astronomy besides the equatorial one,[3] the term and the names of the twelve signs are today mostly associated with horoscopic astrology.[4] The term "zodiac" may also refer to the region of the celestial sphere encompassing the paths of the planets corresponding to the band of about eight arc degrees above and below the ecliptic. The zodiac of a given planet is the band that contains the path of that particular body; e.g., the "zodiac of the Moon" is the band of five degrees above and below the ecliptic. By extension, the "zodiac of the comets" may refer to the band encompassing most short-period comets.[5]

The division of the ecliptic into the zodiacal signs originates in Babylonian ("Chaldean") astronomy during the first half of the 1st millennium BC, likely during Median/"Neo-Babylonian" times (7th century BC).[6] The classical zodiac is a modification of the MUL.APIN catalogue, which was compiled around 1000 BC. Some of the constellations can be traced even further back, to Bronze Age (Old Babylonian) sources, including Gemini "The Twins", from MAŠ.TAB.BA.GAL.GAL "The Great Twins", and Cancer "The Crab", from AL.LUL "The Crayfish", among others.

Babylonian astronomers at some stage during the early 1st millennium BC divided the ecliptic into twelve equal zones of celestial longitude to create the first known celestial coordinate system: a coordinate system that boasts some advantages over modern systems (such as the equatorial coordinate system). The Babylonian calendar as it stood in the 7th century BC assigned each month to a sign, beginning with the position of the Sun at vernal equinox, which, at the time, was depicted as the Aries constellation ("Age of Aries"), for which reason the first sign is still called "Aries" even after the vernal equinox has moved away from the Aries constellation due to the slow precession of the Earth's axis of rotation.[7]

Because the division was made into equal arcs, 30º each, they constituted an ideal system of reference for making predictions about a planet's longitude. However, Babylonian techniques of observational measurements were in a rudimentary stage of evolution and it is unclear whether they had techniques to define in a precise way the boundary lines between the zodiacal signs in the sky.[8] Thus, the need to use stars close to the ecliptic (±9º of latitude) as a set of observational reference points to help positioning a planet within this ecliptic coordinate system.[9] Constellations were given the names of the signs and asterisms could be connected in a way that would resemble the sign's name. Therefore, in spite of its conceptual origin, the Babylonian zodiac became sidereal.[10]

In Babylonian astronomical diaries, a planet position was generally given with respect to a zodiacal sign alone, less often in specific degrees within a sign.[11] When the degrees of longitude were given, they were expressed with reference to the 30º of the zodiacal sign, i.e., not with a reference to the continuous 360º ecliptic.[12] To the construction of their mathematical ephemerides, daily positions of a planet were not as important as the dates when the planet crossed from one zodiacal sign to the next.[13]

Particularly important in the development of Western horoscopic astrology was the astrologer and astronomer Ptolemy, whose work Tetrabiblos laid the basis of the Western astrological tradition.[22] Under the Greeks, and Ptolemy in particular, the planets, Houses, and signs of the zodiac were rationalized and their function set down in a way that has changed little to the present day.[23] Ptolemy lived in the 2nd century AD, three centuries after the discovery of the precession of the equinoxes by Hipparchus around 130 BC. Hipparchus's lost work on precession never circulated very widely until it was brought to prominence by Ptolemy,[24] and there are few explanations of precession outside the work of Ptolemy until late Antiquity, by which time Ptolemy's influence was widely established.[25] Ptolemy clearly explained the theoretical basis of the western zodiac as being a tropical coordinate system, by which the zodiac is aligned to the equinoxes and solstices, rather than the visible constellations that bear the same names as the zodiac signs.[26]

The Hindu zodiac uses the sidereal coordinate system, which makes reference to the fixed stars. The Tropical zodiac (of Mesopotamian origin) is divided by the intersections of the ecliptic and equator, which shifts in relation to the backdrop of fixed stars at a rate of 1° every 72 years, creating the phenomenon known as precession of the equinoxes. The Hindu zodiac, being sidereal, does not maintain this seasonal alignment, but there are still similarities between the two systems. The Hindu zodiac signs and corresponding Greek signs sound very different, being in Sanskrit and Greek respectively, but their symbols are nearly identical. For example, dhanu means "bow" and corresponds to Sagittarius, the "archer", and kumbha means "water-pitcher" and corresponds to Aquarius, the "water-carrier".

The zodiac is found in mediaeval stained glass as at Angers Cathedral, where the master glassmaker, André Robin, made the ornate rosettes for the North and South transepts after the fire there in 1451.[27]

An example of the use of signs as astronomical coordinates may be found in the Nautical Almanac and Astronomical Ephemeris for the year 1767. The "Longitude of the Sun" columns show the sign (represented as a digit from 0 to and including 11), degrees from 0 to 29, minutes, and seconds.[28]

The zodiacal symbols are Early Modern simplifications of conventional pictorial representations of the signs, attested since Hellenistic times.

What follows is a list of the twelve signs of the modern zodiac (with the ecliptic longitudes of their first points), where 0° Aries is understood as the vernal equinox, with their Latin, Greek, Sanskrit, and Babylonian names (but note that the Sanskrit and the Babylonian name equivalents denote the constellations only, not the tropical zodiac signs). Also, the "English translation" is not usually used by English speakers. The Latin names are standard English usage.

Equirectangular plot of declination vs right ascension of the modern constellations with a dotted line denoting the ecliptic. Constellations are colour-coded by family and year established. (detailed view)

The zodiacal signs are distinct from the constellations associated with them, not only because of their drifting apart due to the precession of equinoxes but also because the physical constellations take up varying widths of the ecliptic, so the Sun is not in each constellation for the same amount of time.[31]:25 Thus, Virgo takes up five times as much ecliptic longitude as Scorpius. The zodiacal signs are an abstraction from the physical constellations, and each represent exactly one twelfth of the full circle, or the longitude traversed by the Sun in about 30.4 days.[32]

Some "parazodiacal" constellations are also touched by the paths of the planets. The MUL.APIN lists Orion, Perseus, Auriga, and Andromeda. Furthermore, there are a number of constellations mythologically associated with the zodiacal ones : Piscis Austrinus, The Southern Fish, is attached to Aquarius. In classical maps, it swallows the stream poured out of Aquarius' pitcher, but perhaps it formerly just swam in it. Aquila, The Eagle, was possibly associated with the zodiac by virtue of its main star, Altair.[citation needed]Hydra in the Early Bronze Age marked the celestial equator and was associated with Leo, which is shown standing on the serpent on the Dendera zodiac.[citation needed]Corvus is the Crow or Raven mysteriously perched on the tail of Hydra.

Due to the constellation boundaries being redefined in 1930 by the International Astronomical Union, the path of the ecliptic now officially passes through thirteen constellations: the twelve traditional 'zodiac constellations' plus Ophiuchus, the bottom part of which interjects between Scorpio and Sagittarius. Ophiuchus is an anciently recognized constellation, catalogued along with many others in Ptolemy's Almagest, but not historically referred to as a zodiac constellation.[33][34] The inaccurate description of Ophiuchus as a sign of the zodiac drew media attention in 1995, when the BBC Nine O'Clock News reported that "an extra sign of the zodiac has been announced by the Royal Astronomical Society".[35] There had been no such announcement, and the report had merely sensationalized the 67-year-old 'news' of the IAU's decision to alter the number of designated ecliptic constellations.[36][37]

The theoretical beginning of Aries is the moment of vernal equinox, and all other dates shift accordingly. The precise Gregorian times and dates vary slightly from year to year as the Gregorian calendar shifts relative to the tropical year.[38] These variations remain within less than two days' difference in the recent past and the near-future, vernal equinox in UT always falling either on 20 or 21 March in the period of 1797 to 2043, falling on 19 March in 1796 the last time and in 2044 the next.[39]

Because the Earth's axis is at an angle, some signs take longer to rise than others, and the farther away from the equator the observer is situated, the greater the difference. Thus, signs are spoken of as "long" or "short" ascension.[42]

Western astrology takes the tropical approach, whereas Hindu astrology takes the sidereal one. This results in the originally unified zodiacal coordinate system drifting apart gradually, with a clockwise(westward) precession of 1.4 degrees per century.

For the tropical zodiac used in Western astronomy and astrology, this means that the tropical sign of Aries currently lies somewhere within the constellation Pisces ("Age of Pisces").

The sidereal coordinate system takes into account the ayanamsa, ayan meaning transit or movement, and amsa meaning small part, i.e. movement of equinoxes in small parts. It is unclear when Indians became aware of the precession of the equinoxes, but Bhaskar II's 12th-century treatise Siddhanta Shiromani gives equations for measurement of precession of equinoxes, and says his equations are based on some lost equations of Suryasiddhanta plus the equation of Munjaala.

The discovery of precession is attributed to Hipparchus around 130 BC. Ptolemy quotes from Hipparchus’ now lost work entitled “On the Displacement of the Solstitial and Equinoctial Points” in the seventh book of his 2nd century astronomical text, Almagest, where he describes the phenomenon of precession and estimates its value.[24] Ptolemy clarified that the convention of Greek mathematical astronomy was to commence the zodiac from the point of the vernal equinox and to always refer to this point as “the first degree” of Aries.[25] This is known as the “tropical zodiac” (from the Greek word trópos, turn)[45] because its starting point revolves through the circle of background constellations over time.

The principle of the vernal point acting as the first degree of the zodiac for Greek astronomers is also described in the 1st century BC astronomical text of Geminus of Rhodes. Geminus explains that Greek astronomers of his era associate the first degrees of the zodiac signs with the two solstices and the two equinoxes, in contrast to the older Chaldean (Babylonian) system, which placed these points within the zodiac signs.[25] This illustrates that Ptolemy merely clarified the convention of Greek astronomers and did not originate the principle of the tropical zodiac, as is sometimes assumed.

Ptolemy also demonstrates that the principle of the tropical zodiac was well known to his predecessors within his astrological text, the Tetrabiblos, where he explains why it would be an error to associate the regularly spaced signs of the seasonally aligned zodiac with the irregular boundaries of the visible constellations:

The beginnings of the signs, and likewise those of the terms, are to be taken from the equinoctial and tropical points. This rule is not only clearly stated by writers on the subject, but is also especially evident by the demonstration constantly afforded, that their natures, influences and familiarities have no other origin than from the tropics and equinoxes, as has been already plainly shown. And, if other beginnings were allowed, it would either be necessary to exclude the natures of the signs from the theory of prognostication, or impossible to avoid error in then retaining and making use of them; as the regularity of their spaces and distances, upon which their influence depends, would then be invaded and broken in upon.[26]

The Sun's placement upon the vernal equinox, which occurs annually around 21 March, defines the starting point for measurement, the first degree of which is historically known as the "first point of Aries". The first 30° along the ecliptic is nominally designated as the zodiac sign Aries, which no longer falls within the proximity of the constellation Aries since the effect of precession is to move the vernal point through the backdrop of visible constellations (it is currently located near the end of the constellation Pisces, having been within that constellation since the 2nd century AD).[48] The subsequent 30° of the ecliptic is nominally designated the zodiac sign Taurus, and so on through the twelve signs of the zodiac so that each occupies 1/12th (30°) of the zodiac's great circle. Zodiac signs have never been used to determine the boundaries of astronomical constellations that lie in the vicinity of the zodiac, which are, and always have been, irregular in their size and shape.[46]

The convention of measuring celestial longitude within individual signs was still being used in the mid-19th century,[49] but modern astronomy now numbers degrees of celestial longitude from 0° to 360°, rather than 0° to 30° within each sign.

There are many mnemonics for remembering the 12 signs of the zodiac in order. A traditional mnemonic:[50]

The ram, the bull, the heavenly twins,
And next the crab, the lion shines,
The virgin and the scales,
The scorpion, archer, and the goat,
The man who holds the watering-pot,
And fish with glittering scales.

^The notion received further international media attention in January 2011, when it was reported that astronomer Parke Kunkle, a board-member of the Minnesota Planetarium Society, had suggested that Ophiuchus was the zodiac's '13th sign'. He later issued a statement to say he had not reported that the zodiac ought to include 13 signs instead of 12, but was only mentioning that there were 13 constellations; reported in Mad Astronomy: Why did your zodiac sign change? 13 January 2011.

^The Gregorian calendar is built to satisfy the First Council of Nicaea, which placed vernal equinox is on 21 March, but it is not possible to keep it on a single day within a reasonable system of leap days.